In power converters, losses appear as none of the components has ideal characteristics. The losses introduce heat in the power circuitry, which apart from consuming energy introduces thermal strain to all components, reducing their life span.
It is desirable to increase the frequency of operation of power converters as the generated output then can be more exactly controlled. Increasing the switching frequency leads to lower switching ripple, and smaller component values which in turn leads to a more compact, light-weight and cost-effective implementation of the invention. Further, lower switching ripple allows for potentially lowered EMI, which aligns with the goal of a non-disturbing switch. Still further, having a high switching frequency allows for higher frequency currents to be generated by the power converter, widening the range of applications suitable for the converter.
However, increasing the frequency also increases switching losses, as most losses appear on switching cycle basis. Forcing the transistor to commutate while a current is flowing through it or when there is a potential difference over it, requires energy which must be supplied to the gate of the transistor. Thus, reducing the current through the transistor or the voltage thereover reduces the total power input for the switch and thus the total power input to the system.
One way of reducing the losses over a particular switch is to ad a resonant component to the circuitry in which a current is generated by an inductive element, by the discharge of a capacitor. A circuit employing this technique is known as a resonant converter, and the method of using resonance to facilitate commutation is known as soft switching. There are generally two types of soft-switching: low-voltage switching and low-current switching. Low-voltage switching involves minimizing the voltage or potential difference over the switch prior to commutation, whereas low-current switching involves minimizing the current through the switch prior to switching.
One soft switching solution is provided in U.S. Pat. No. 5,047,913 (to De Doncker et al.). De Doncker suggests using controlled switches in the resonant auxiliary circuitry for overcoming the problem of active device switching losses in power converters. The reduction of losses in the power converters enables operation at higher switching frequencies. De Doncker describes that the resonant output voltage may fall short of the opposite rail voltage due to component resistances, device conduction losses and inadequate forcing potential. As a result, the next switching device in the inverter pole to be turned on may be switched at the peak of the resonating voltage, and hence must absorb some switching losses due to the non-zero voltage turn-on, including the energy dump from the parallel capacitor.
All switching loads cause electromagnetic interference (EMI), and in high voltage applications, such as active filters, the EMI is particularly large. Regulations demand that electronics do not emit EMI over certain values, and generating less EMI is therefore an important goal in its own right. In applications where the converter or inverter is connected directly to the grid, EMI noise can cause problems which are normally solved by employing Electro Magnetic Compatibility (EMC) filters. EMC filters must be placed in series with the converter, thereby handling the full current capacity. By minimizing the EMI, EMC filters can be eliminated from the converter design, which reduces the size and cost of the circuitry.
A resonant converter comprises two main switching devices per phase. The switching devices have diodes connected in parallel therewith. The resonant converter further comprises an auxiliary resonant commutation circuit including auxiliary switching devices coupled in series with an inductor and a capacitor. When a diode is switched from a non-conducting to a conducting state and vice versa is has an intrinsic recovery time due to the charge carriers stored in the diode, during this recovery time, the diode can conduct in the reverse direction as the diode does not attain its blocking capability until the charge in the junction is depleted. The reverse recovery time is typically in the range 10-1000 ns during which time a reverse recovery current flows through the diode in the reverse direction.
The reverse recovery current brings increased EMI noise as the reverse recovery current along with the reactive elements of the circuitry creates harmonics, the effect can be significant when switching large currents at high frequency. To decrease the amount of losses in the system and increase the switching speeds it would be advantageous to have a resonant converter in which the problem with the reverse recovery current of the diodes is reduced.